The present invention generally relates to thermal printing apparatuses which include heat generating elements arranged in segments or dots in its printing head and records data on a heat-sensitive colorable paper sheet by selectively energizing the heat generating elements in response to input video data. More particularly, the present invention is directed to a drive circuit for a printing head of the type which has a plurality of heat generating elements divided into some groups and reproduces images corresponding to input video signals by selectively supplying the heat generating elements with voltages on a group by group basis.
A known printer of the type described comprises a printing head provided with a plurality of groups of heat generating elements (referred to simply as "element groups" hereinafter). It also comprises a drive circuit for selecting desired ones of the element groups in accordance with input video signals and causes a heat-sensitive paper sheet to color by supplying the selected element groups with voltages. The drive circuit includes switching circuits common in number to the element groups so as to feed the voltages to the individual element groups. The output voltages of the switching circuits remain substantially constant despite any change in a power source voltage due to the presence of a Zener diode at the input side thereof for providing a reference voltage. This ensures uniform voltage distributions to the element groups which in turn allows all the elements in the head to generate constant amounts of heat in a stable manner. Such a known thermal printer is practically usable, but not to a satisfactory degree for the following reasons.
The resistance of heat generating elements differs from one recording head to another. The voltage to be coupled to heat generating elements of a given recording head must be determined to duely match with the resistance of the elements. Hence, it is necessary to choose a Zener diode providing a specific reference voltage for each of printing heads though the latter may resemble each other. Also, when a recording head is replaced with another, the reference voltage needs be changed in conformity with the new recording head. Replacement of a Zener diode for each head or selection of a Zener diode having a desired reference voltage V.sub.Z is quite troublesome and sometimes hardly practicable.